Organism Onchocerca volvulus, a nematode worm that has a predilection for the skin and eye, is transmitted by Simulium flies. Microfilariae are taken up by the fly when it bites humans and then undergo larval changes within the thoracic muscles, migrating to the head of the fly as infective larvae. When the fly bites again, microfilar-iae break out on to the skin to enter via any abrasion, especially the bite wound.

Clinical features The microfilariae as they migrate through the skin cause itching and damage resulting in skin changes, such as hanging groin and discoloration, the so-called 'leopard skin'. They migrate through the skin and also enter the eye, where the reaction caused by their death leads to eye damage, the person in the course of time becoming blind, giving onchocerciasis its other name 'river blindness'.

Diagnosis is made by taking skin-snips, which are placed in saline and the liberated microfilariae identified (Fig. 15.9). Taking a measured area of skin with a special punch allows density measurements to be made. A slit-lamp examination of the eye may reveal microfilariae in the anterior chamber or characteristic eye damage. The adult worms live in palpable nodules in the skin, so their presence and characteristic skin changes can suggest a clinical diagnosis.

Transmission The vector Simulium, also called the black fly, breeds in fast-flowing streams where it is found in large numbers. The female fly attaches its eggs to the leaves of water plants on which they develop. The larvae require high oxygen levels, so are found only in fast-flowing water (Fig. 15.12). The fly has a painful bite and is persistent making it a considerable nuisance, but it is also a powerful flier and assisted by the wind can travel up to 100 km in search of a blood meal.

The Simulium vectors and their usual breeding places are listed in Table 15.4, the African flies preferring to bite the lower body, whereas the South American flies attack the upper part of the body. Although they can fly great distances, maximum density is at the breeding place, resulting in focal infection. They are outdoor, daytime biters, but each species prefer different times of day to seek their blood meal. South American Simulium have pharyngeal armatures, whereas African species do not, but mortality due to superinfection by Onchocerca is not important. The adult flies live for 2-3 weeks (with a maximum of 3 months), but prefer to feed on animals rather than humans. However, people need to collect water so it is when they have to come to the river, to wash or collect drinking water that they stand the greatest chance of becoming infected.

O. volvulus only infects humans (and epidemiologically insignificant chimpanzees and gorillas). Eye and skin pathology is related to the proximity of the nodules, so when there are more nodules on the upper part of the body, there is a higher prevalence of blindness. In Africa, the savannah infection produces more blindness than that acquired in forests.

Microfilariae are found only in the skin, a high density leading to the more severe clinical manifestations as well as producing greater opportunity to infect flies. They survive for up to 2.5 years and have a periodic cycle with a peak at 1600-1800 hours, but this is relatively unimportant.

Incubation period is prolonged, normally taking about 1 year for symptoms to start following infection.

Lalat Simulium Vector Filariasis

Fig. 15.12. Simulium the vector of onchocerciasis. (a) Adult. (b) Larvae and a pupa attached to a water plant, the stream flowing in the direction of the arrow.

Period of communicability is for some 16-17 years, adult worms producing micro-filariae into old age. Simulium becomes infective after 6-13 days, depending on temperature.

Occurrence and distribution Onchocerciasis is found only in tropical Africa, Yemen and in South and Central America, with well-marked foci in much of this area (Fig. 15.13). In West and much of Central Africa, the infection is more widespread with the most westerly part of the region covered by the Onchocerciasis Control Programme (see below).

Repeat infection and progressive damage from dying microfilariae means that blindness is more common in adults, children then having to lead them around until their turn to become blind. Because of these severe consequences, abandonment of good village sites close to rivers has frequently resulted, although control programmes have largely reversed this trend.

Table 15.4. Simulium (S.) vectors of onchocerciasis. CAR, Central African Republic.

Geographical area


Breeding place


West Africa, CAR, Sudan,

S. damnosum

Large rivers

Savannah, but

Uganda, Ethiopia, (Yemen?)

species complex

sometimes fore

West Africa, CAR, Sudan

S. sirbanum

Large rivers


West Africa, CAR, Congo

S. squamosum

Small to medium-sized

Forest savannah,

rivers in hilly areas


West Africa

S. soubrense

Large rivers

Forest, savannah

West Africa

S. sanctipauli

Large rivers


West Africa

S. yahense

Small watercourses


Cameroon, CAR, Tanzania

S. mengense

Large rivers


Congo, Burundi, Uganda,

S. kilibanum

Large rivers


Tanzania, Malawi

Congo, Burundi, Rwanda,

S. naevi species

Heavily shaded small


Uganda, Sudan


permanent rivers in forest


S. ethiopiense

Heavily shaded, small


permanent rivers in forest


S. woodi

Heavily shaded, small


permanent rivers in


Guatemala, Mexico

S. ochraceum

Small mountain streams


Guatemala, Mexico,

S. metallicum

Small streams



Colombia, Ecuador, Venezuela

S. exiguum

Large rivers


Brazil, Venezuela

S. guianense

Large, fast-flowing rivers


Brazil, Venezuela

S. oyapockense

Large rivers


Control and prevention Similar to lymphatic filariasis, various approaches to control can be tried. These are:

• reducing the fly density;

• avoidance of fly breeding places;

• reducing the microfilarial density;

• reduction of the number of adult worms;

• reduction in the number of Simulium bites.

reducing the fly density (larviciding) The larvae breed in water, so insecticide is sprayed on streams and rivers. The larvae are relatively sensitive to insecticides so low-dose applications, 0.05-0.1 mg/l are effective. Temephos (Abate) is suitable as it is effective in a very low dose, is relatively non-toxic to fish and retains some residual action. It exerts its effect for some 20-40 km downstream in the wet season. The main difficulty with larviciding is to ensure that every water course is treated. Owing to the flies' ability to cover large distances, re-colonization soon takes place when in-secticidal applications are discontinued. Although expensive, the extra cost of using aircraft and helicopters can be justified if many water courses, spread over large areas of countryside, have to be covered.

Unfortunately, insecticidal resistance has occurred in a number of areas, so biological control with B. thuringiensis is an alternative. This does not have the spreading power of insecticides and greater concentrations need to be used (in the order of 0.9 mg/l) and has to be mixed with water before it can be applied.

avoidance of fly breeding places Maximum contact between humans and flies occurs near rivers where Simulium breed, but ggg Area covered by the OCP

Onchocerciasis Cases

Fig. 15.13. Distribution of onchocerciasis. (a) Africa and Yemen. Continued Overleaf.

ggg Area covered by the OCP

Fig. 15.13. Distribution of onchocerciasis. (a) Africa and Yemen. Continued Overleaf.

NJ lo

The Americas
Fig. 15.13 (cont'd). Distribution of onchocerciasis. (b) the Americas. OCP, former Onchocerciasis Control Project area. (From World Health Organization Technical Report Series (1995) Onchocerciasis and Its Control, No. 852. Reproduced, by permission of the World Health Organization, Geneva.)

these can be avoided by providing alternative water sources, such as wells or a piped supply to nearby villages.

reducing the microfilarial density Ivermec-

tin immobilizes microfilariae, which are flushed out of the skin and eye and killed in the lymph nodes. As microfilarial death occurs away from the skin and eye, irritation is minimized and ocular reaction reduced. It can be given as a single dose of ivermectin 200mcg/kg with re-treatment at 6- and 18-month intervals. This means that mass therapy for onchocerciasis can be used as an adjunct to vector control.

reducing the number of adult worms Since the adult worms live for a considerable period of time, during all of which they are producing microfilariae, specific attack on the adult parasites can reduce both the symptoms and potential for transmitting infection. Nodulectomy or the surgical removal of adult worms from skin nodules can be a relatively effective procedure, practised particularly in the Guatemala onchocercal areas where nodules, more common in the upper parts of the body, are likely to produce ocular lesions. Moxi-dectin, used in veterinary medicine, has been found to kill adult worms, so is likely to be used in the treatment of the individual and community if free of side-effects.

reducing the number of SlMULIUM bites Personal protection is less effective against Simulium than with mosquitoes, with nets being inappropriate, although repellents have some effect. The wearing of long-sleeved shirts and long trousers with a hat and net can be used by individuals investigating the disease, but are not methods that can be developed for mass use. Avoiding passage through breeding sites will reduce fly biting.

onchocerciasis control programmes Adult O. volvulus can live for 15-17 years, so any control programme would need to be main tained for this length of time before eradication can take place. However, most programmes seek to reduce the intensity of infection to a level where symptoms are absent. The criteria used in the Onchocer-ciasis Control Project (OCP) in West Africa were:

• less than 100 infective larvae/person/ year;

• annual biting rates of less than 1000.

After many years of operation, the OCP programme finished in 2002, with delegation to individual countries to detect and treat all new cases.

The main method of control is larvicid-ing, which can be extremely effective if carried out thoroughly. Species eradication of S. naevi was achieved in Kenya by methodically treating every water course with DDT. Where the disease covers a limited area, such an intense programme could be considered. In a more diffuse focus, the borders of control need to be extended sufficiently to prevent re-invasion by Simulium flying in from outside. While resistance is a serious problem, resistant Simulium are less important in transmission.

Mass drug therapy, or selective treatment to persons with heavy infections, can be given right from the start of the programme. This will rapidly reduce the microfilariae level and the potential for infecting flies. Preventing blindness (with ivermectin) has been particularly valuable in obtaining the cooperation of people. As lymphatic filariasis and onchocerciasis occur in the same areas in a number of countries (mainly in West Africa) and ivermectin is used to treat both diseases, joint programmes (with the addition of albendazole) are cost-effective. Moxidectin, which remains in the plasma for a much longer period (20 days as opposed to ivermectin's 2 days), could permit more flexible treatment regimes.

Treatment Ivermectin has been very effective, especially in the reduction of blindness, but moxidectin with its killing effect on adult worms may produce a more radical cure.

Surveillance Skin and nodule surveys can be used to indicate areas that need more intense skin-snip examination (see also the OCP programme above).

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